Demand for batteries with higher energy densities at lower costs is fueling the search for alternatives to traditional nickel-cadmium or lead-acid electrochemical cells. For example, nickel-metal hydride electrochemical cells have greater energy densities than nickel-cadmium cells but are correspondingly more expensive. Zinc-air cells offer greater energy densities as well but cannot be rapidly charged and have shorter operational life spans. Lithium-ion type cells are compact, light, offer high energy densities, have a high discharge voltage and exhibit stable discharge characteristics. Lithium-ion cells are well suited to applications requiring current draw for extended periods of time and are relatively inexpensive.
Traditionally, lithium type batteries have not been widely utilized as a main power source in the portable electronics industry because of the inherent characteristics of lithium electrochemical cells that present engineering difficulties. Effective and efficient charging of lithium cells, especially when utilized in a series configuration, is often difficult to accomplish. Manufacturers of lithium type electrochemical cells typically dictate stringent charging conditions to maximize the charge and to prevent damage caused by overcharging. The maximum charge voltage and current applied to the cells must be limited to predetermined levels.
Additionally, lithium electrochemical cells are susceptible to self-discharge due to the internal presence of hydrogen in the electrochemical cells. For example, when a battery pack comprises multiple series connected lithium cells, the voltages of the cells are typically mismatched due to the varying presence of internal hydrogen. This voltage mismatch causes the battery pack voltage to be lower than its nominal value and results in rapid loss of electric charge. During charging of the battery, the charging process may completely charge some of the cells and completely discharge other cells. The mismatched cells ultimately fail and thereby cause the instability and premature failure of the battery pack. Additionally, because some of the lithium cells are completely charged while others are discharged, the resulting total series voltage of the lithium cells is less than the nominal fully charged voltage of the battery back thereby causing the battery pack to be overcharged. Overcharging of the battery pack ultimately leads to premature failure and may possibly result in a fire or explosion of the electrochemical cells.
It is for these mentioned reasons that lithium electrochemical cells have not been widely utilized as the main power source in portable battery powered electronic equipment. The charging process is complex and requires that the charging process is carefully monitored and controlled so that the charging voltage and current is strictly limited. Additionally, the lithium battery charging apparatus must account for any voltage mismatch between each of the individual cells. Thus, the battery charger and battery pack typically requires complicated construction and complex, expensive charging circuitry.